Safety concerns such as inflammation are essential problems for lithium batteries incorporating flammable organic solvent electrolytes. A radical solution to this problem of safety is to use a nonflammable electrolyte to replace the flammable organic solvent electrolyte. Representative examples of the nonflammable electrolyte are inorganic lithium ion-conducting solid electrolytes. Use of inorganic solid electrolytes not only enhances the safety but also allows integration with electronic circuits since the batteries can be formed as thin films. Moreover, since inorganic solid electrolytes have ion-selectivity, the reliability of batteries, such as cycle lifetime and storage lifetime, can be improved.
The cause of most of self-discharge and a decrease in capacity accompanying charge-discharge cycle is the side reaction inside the batteries. In lithium batteries, in particular, lithium ion batteries, the ions that contribute to the electrode reaction are lithium ions only. However, in liquid electrolytes, anions, solvent molecules, and impurities also migrate and diffuse in positive electrode surfaces having high oxidizing power or negative electrode surfaces having high reducing power, thereby sometimes resulting in oxidation or reduction. Such side reactions degrade the battery performance.
In contrast, inorganic solid electrolytes have ion selectivity. That is, only lithium ions move in the lithium ion-conducting inorganic solid electrolyte. Thus, unlike in the liquid electrolytes, continuation of side reactions caused by diffusion of matters other than lithium ions in the electrode surfaces is prevented. Therefore, all-solid batteries incorporating inorganic solid electrolytes have long life and achieve low self-discharge.
The present inventors have previously invented a method in which a carbon material having a low potential and a high capacity density is used as the negative electrode material of an all-solid lithium secondary battery (patent document 1) and succeeded in increasing the energy density of the all-solid lithium secondary battery (patent document 2). However, the output density obtained from the all-solid lithium secondary batteries was only about several hundred microamperes per square centimeter, which was still low compared to that achieved by liquid electrolyte systems.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-68361
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-217663